1. Field of the Invention
The present invention relates to a wavelength selective switch.
2. Description of the Related Art
A basic structure of a wavelength selective switch has been disclosed in Japanese Patent No. 3937403, in the name of optical add-drop multiplexer.
Here, the wavelength selective switch is a device to be placed in a node in an ROADM (reconfigurable optical add-drop multiplexer) (a system or a technology used in a large-capacity network in which, a wavelength-multiplexed optical signal is add/drop an optical signal as it is), and is an optical switch which carries out for each wavelength, switching of a transmission path of a light signal which has been wavelength-multiplexed.
By the wavelength selective switch in each node, it is possible to select an optical signal of an arbitrary wavelength from the optical signal which has been wavelength-multiplexed, and to combine light of an arbitrary wavelength with the optical signal which has been wavelength-multiplexed. In this wavelength selective switch, a diffraction grating is used as a dispersive element.
Moreover, a light dispersing device which includes a diffraction grating as a dispersive element, which is insensitive to temperature, and of which, an output angle does not change, has been disclosed in Unexamined Patent Publication (Japanese Patent Application Laid-open Publication) No. 2003-509714.
In Japanese Patent Application Laid-open Publication No. 2003-509714, changing of dispersion characteristics according to a change in the temperature has been raised as an issue, and a countermeasure has been taken. Japanese Patent Application Laid-open Publication No. 2003-509714 has been shown a change in the dispersion characteristics according to the change in the temperature is caused by a thermal expansion of the diffraction grating. However, in a large number of cases, a change in a refractive index of air, and a change in a refractive index of the dispersive element due to the change in the temperature when light pass through the dispersive element are a main cause of the change in dispersion characteristics. When the dispersion characteristics change, there is a disadvantage that a frequency band which is controlled by respective deflecting elements in the wavelength selective switch changes due to the change in the temperature.
Particularly, the change in the dispersion characteristics is substantial when a dispersive element in which, a material having a high refractive index is used. Due to such reason, in the conventional wavelength selective switch, it was difficult to have stable dispersion characteristics with respect to the change in the temperature.